Noel S. Weiss, MD, DrPH; V. Paul Doria-Rose, DVM, PhD
Potential Financial Conflicts of Interest: None disclosed.
Weiss N., Doria-Rose V.; The Effectiveness of Colonoscopy in Reducing Mortality From Colorectal Cancer. Ann Intern Med. 2009;150:817-818. doi: 10.7326/0003-4819-150-11-200906020-00015
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Published: Ann Intern Med. 2009;150(11):817-818.
TO THE EDITOR:
To provide information regarding the efficacy of screening colonoscopy in preventing death from colorectal cancer, Baxter and colleagues (1) analyzed administrative claims data from Ontario, Canada. Although this choice had the advantage of yielding a very large sample (10 292 deaths from colorectal cancer), it had some drawbacks that we believe seriously threaten the validity of the results obtained.
First, colonoscopy, like most screening tests for cancer, can also be used to evaluate persons who are suspected to have cancer, on the basis of symptoms or signs. Because the case patients in this case–control study all had colorectal cancer, many of them would have received colonoscopy in response to symptoms or signs. The data available to Baxter and colleagues could not distinguish between screening and diagnostic colonoscopies. To the extent that tests not screening done for were labeled as “screening,” the proportion of “screened” cases would be spuriously high, leading to a spuriously high odds ratio and thus a spuriously low estimate of screening efficacy.
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